For the past week my expansion tank has been filling with hot water causing it to overflow. It is a fully pumped system. The water seems to be coming from the pipe connected to the bottom of the expansion tank. I've checked the ball cock and this definately isn't the problem. A plumber I have had look at the system has fitted a check valve to this pipe, but won't this stop the system being able to expand?

I assume that what you call the expansion tank is the cold water storage tank that supplies the water to the hot water cylinder where it is heater by a heat exchanger off your boiler or an immersion heater? Should the water in the hot water cylinder become too hot it expands and there is an overflow pipe back to the cold water storage tank. The cold water tank in turn has a overflow pipe to the outside of the building.

I can think of two things to check, one is the balance of water in the system. If there is too much water then when hot water returns to the cold water tank it will spill out then when it cools the ball cock will sink too low and draw in fresh cold water only for the same cycle to happen all over again when the water becomes too hot and expands. Try lowerring the water level at which the ball cock draws water in.

The other thing I would want to check is why is the water overheating? After all it costs a lot of money to heat this water up! Is the thermostat working correctly? If it is an immersion heater I would replace with a new type thermostat with the extra reset button to be on the safe side. Remember the recent news about a child being scaled to death because of a faulty thermostat?

With regard to the check valve, I am an electrician so not sure what this term means but if it obstructs the flow of the water from the hot water cylinder to the header tank then there is the risk of pressure building up and the cylinder exploding. I can not think of a reason for putting any kind og valve in this safety feature.

I'm no expert, so bear with me! The system would appear to consist of a cold water storage tank that supplies the hot water cylinder, but as well as this there is a smaller tank that sits next to it, which I think is called the F&E tank. It is this one that is overflowing. There is a pump and a 3-way valve that feeds the central heating/hot water cylinder, and on the outlet of the cylinder coil the pipework tees into the central heating pipework, then goes back to the boiler. It is just before this that a pipe is taken off and goes into the bottom of the Feed & Expansion tank. It is this pipe which the plumber fitted the check valve (one way valve) so I can see how it feeds the system, I just can't see how it allows for expansion.

Thatâ€™s not a plumber !!! if you are talking about the Feed & Expansion tank (F&E), you MUST NOT fit or have any device to close the â€˜primary return pipe, or the â€˜open vent pipeâ€™, from the Primary flow that loops up and exits above the F&E tank.

It all depends on the system you have:

If you have a â€˜indirect single-feedâ€™ system (an air bubble in the HWC) the HWC may/will have lost its air bubble, drain down the HWC and refill this will reinstate the air bubble.

You say itâ€™s â€˜fully pumpedâ€™, the problem will be overheating of the system, check the thermostats are working correctly.

you could also check the â€˜motorised valvesâ€™ first (two or three port valves)

If it is your cold water storage cistern Greenhm is correct,

There are three pipes that connect to the cold water storage cistern (CWSC). One at the top that feeds the main water in, controlled by a â€˜float operated valveâ€™. The â€˜open vent pipeâ€™ that loops above the tank and comes from the boiler primary flow, this keeps the whole system at atmospheric pressure, and the one (possibly two) at the bottom of the CWSC which is the cold feed, going to cold taps and the hot water cylinder (HWC).

You turn on a hot tap and you get hot water, this draw off, draws cold water from the CWSC. As the CWSC is above the tap, the pressure (head) of the water is being pushed through the HWC to the tap. On its way the water is heated in the HWC, water expands by 4% when it is heated, (250lt HWC = 10lt). If you close the tap and close the valve fitted by the said plumber, you now have a pressure cylinder.

The safety aspects of the system are, if the thermostat fails to cut out the heat (immersion or boiler) the extra expanded hot water will flow back up the â€˜cold feed pipeâ€™ and into the CWSC, if the HWC boils the steam will exit up the â€˜open vent pipeâ€™.

If you or anyone else fits a valve to either the â€˜open vent pipeâ€™ or the â€˜COLD FEED PIPEâ€™ you have invented a bomb and when you turn on the heat the HWC will explode.

Check the immersion heater and as Greenhm said fit a safety cut-out thermostat.

Do NOT lower the water level at which the ball cock draws water in.

The problem will be overheating of the HWC,

Check the thermostat is set and or is working correctly?

If it is an immersion heater replace with a new high temp cut out thermostat with reset button

Ask the plumber for a refund after he has removed the valves, if you let him in!

Hello Skids, appreciate the warning! It is the Feed & Expansion tank (F&E), and from what i've read, it does seem to be a fully pumped system, although i'm not sure. There is one pipeline going into the HW cylinder from the pump, and this then leaves the bottom of the cylinder and tees into the central heating (I think!) I'll try turning the thermostats (cylinder and boiler) down to see if it helps. I'll also take the check valve out! Thanks for the advice

Get a hosepipe and fill with water; hold the ends level one in each hand, lift the left end & your right shoe gets wet ~ move this logic to the F&E tank

Its an open system (atmospheric pressure) and the water needs to go somewhere when it heats up, the â€˜open vent pipeâ€™ is above the â€˜cold feed pipeâ€™ ~ therefore as the water expands it and will move back into the F&E tank.

Think of a pipe in a ring (hulahoop) containing 100lt of water, in this ring put a boiler, pump and a HWC; boiler heats the water, pump pushes the water around the loop, water expands by 4% = 4lt, no where for the 4lt, pipe or HWC splits ~

Now think of a pipe in the shape of an upside down P, the D contains 100lt of water, add a boiler, pump and a HWC; boiler heats the water, pump pushes the water around the loop (D), water expands to 104lt, the 4lt goes up the leg of the P (I) ~ no explosion ~ safe system

Now add somewhere to put water into the system, a CWSC, somewhere to draw water off, taps, add in a Central Heating loop, and you have a fully pumped system. Donâ€™t forget the 3 port valves so the boiler knows where to send the hot water, taps or Radâ€™s or both.

suspect water mixing in the hot water cylinder, its just trying to reach the same level as that in the cold water tank, I think a new cylinder is required but try turning the cold feed off to this overnight & see if exactly the same level in the morning.